Aqueous methylparaben degradation by dielectric barrier discharge induced non-thermal plasma combined with ZnO-rGO nanosheets

The feasibility of methylparaben (MeP) degradation in aqueous solution by dielectric barrier discharge induced non-thermal plasma combined with zinc oxide-reduced graphene oxide (ZnO-rGO) nanosheets was investigated. The ZnO-rGO nanosheets were synthesized by hydrothermal method and characterized by X-ray diffraction, nitrogen adsorption-desorption isotherm, scanning electron microscope, transmission electron microscope, UV-vis absorption spectroscopy, photocurrent, photoluminescence spectroscopy and X-ray photoelectron spectroscopy. The effects of various factors (discharge power, initial concentration of MeP, initial pH value and air flow rate) on the degradation of MeP were evaluated and the changes of solution pH, conductivity and TOC during the degradation process were discussed. The results showed that when the discharge power was 20 W, the airflow rate was 20 L/h, the ZnO-rGO nanosheets dosage was 0.015 g/L, the initial concentration of MeP was 20 mg/L, and the initial pH value was 7.0, the degradation efficiency of MeP achieved 99% at 15 min. The adding of ZnO-rGO nanosheets to the non-thermal plasma system greatly enhanced the degradation efficiency of MeP. In addition, the role of some reactive species during the degradation process of MeP was also investigated. The degradation process of MeP was well fitted by the pseudo-first-order kinetics. Good stability of the synthetic ZnO-rGO was observed. The degradation intermediates of MeP were determined by gas chromatography-mass spectrometer and the degradation pathways were proposed. Under the attack of reactive species, conjugated structure destruction, hydroxylation, carboxylation and ring-opening reaction occurred during MeP degradation, resulting in the formation of organic acids and alcohols; furthermore, some of the degradation intermediates were mineralized to H2O and CO2.